Modulation of Host Immune Responses by Overexpression of Immunodominant Antigens of Mycobacterium tuberculosis in Bacille Calmette–Guérin

Abstract

Based on their immunodominant nature and ability to induce appropriate immune responses in the host, several antigens of Mycobacterium tuberculosis have shown promise of protection. However, most of the candidate vaccines developed by employing various strategies have afforded protection that is at best comparable with bacillus Calmette-Guérin (BCG) in animal models. Due to the inherent ability of BCG to prime cellular responses in the host, it has become an attractive vehicle for development of a vaccine against intracellular infections. In this study, we have cloned the genes of three immunodominant antigens of M. tuberculosis viz. the ESAT6 (Rv3875), the 19 kDa lipoprotein (Rv3763) and the 38 kDa antigen (Pst homolog) (Rv0934) in pSD5 under the transcriptional control of Trrn, a strong mycobacterial promoter, and expressed them in BCG. The19 kDa antigen and the 38 kDa antigen were expressed at levels that were approximately five and eightfolds higher in the cytosols of recombinant BCG strains rBCG19T and rBCG38T, respectively, as compared with their corresponding levels in M. bovis BCG. Both these antigens were also secreted into the extracellular medium at enhanced levels (19 kDa antigen fourfold and 38 kDa antigen twofold) by rBCG strains in comparison with the wild type BCG. ESAT6 antigen, which is absent in M. bovis BCG, was also expressed at a very high level in the cytosol of the rBCG strain (rBCGE6T). Evaluation of immune responses induced by these three rBCG strains in mice shows a markedly different pattern. The rBCG strain overexpressing the 38 kDa antigen exhibited a predominant T helper 1 (Th1) response with high levels of interferon-g (IFN-g) production, whereas overexpression of the 19 kDa antigen resulted in completely polarized Th2 responses against the BCG sonicate. The rBCG-expressing ESAT6 antigen induced a mixed Th1/Th2 response. Our observations suggest that the 38 kDa antigen may hold excellent promise in the rBCG approach for the development of a vaccine against tuberculosis.

Lactoferrin enhanced efficacy of the BCG vaccine to generate host protective responses against challenge with virulent Mycobacterium tuberculosis

“…BCG antigen stimulated splenocytes from non-immunized mice (38 +/− 17 pg/mL) had similar level of IL-1β compared to mice immunized with BCG/ lactoferrin (44 +/− 21 pg/mL); production of IL-1β was significantly decreased in splenocytes from the BCG immunized group (24 +/− 13 pg/mL) compared to the BCG/lactoferrin group. TB vaccines are useful when given as a one time injection or when administered using various schedules of prime-boost protocols [10,14,15,[43][44][45][46][47][48][49]. The next experiments examined the BCG/lactoferrin saline immunization protocol to generate protection against MTB infection when mice were immunized once (Protocol C), or when boosted prior to infection (Protocol D).…”

“…Many novel vaccine candidates demonstrating superiority over the existing BCG vaccine are genetically modified live BCG recombinants [7,9,10,15,44,47,48], with a general realization of the requirement to maintain the multitude of cross-reactive mycobacterial antigens. In these studies, bovine lactoferrin was used to significantly enhance the current BCG vaccine.…”

Lipoproteins of Bacterial Pathogens

“…In addition to the well-characterized lipoprotein vaccine antigens of Neisseria discussed above, bacterial lipoproteins have been shown to affect both the innate and acquired immune systems via such mechanisms as TLR2 signaling and the generation of cytotoxic T lymphocytes and bactericidal antibodies (11,104,204). As such, a number of lipoproteins from many pathogens have been evaluated as vaccine candidates (6,48,99,136,143,152,193). Many of these, however, have been administered in recombinant form, making it difficult to ascertain the contribution of the native acyl moiety to protection.…”

Vaccines for Tuberculosis: Novel Concepts and Recent Progress

“…While there is no empirical evidence to suggest it is a problem in human disease, the presence of the 19-kDa gene-and other possibly antiprotective proteins-in live mycobacterial vaccines remains a concern. At the very least, such proteins are of dubious value as vaccine candidates (127,191). Taken in total, these studies indicate that M. tuberculosis is able to interfere with almost every stage of the host's immune response (Fig.…”

“…The improved efficacy of rBCG30 came as something of a surprise, since BCG possesses a functional gene for Ag85B. However, recent work has shown that overexpression of genes can alter the immune response to the antigens they encode (127), reminding us that the mere presence of a gene tells us little about its expression in vivo and that a gene's effects may be modulated by the presence or absence of other genes. The second piece of supporting evidence is the observation that new vaccines based on attenuation of Mycobacterium bovis (the parental strain of BCG) are both more virulent and give greater protective efficacy than BCG (34).…”